Testing locations for the ACT are found across the globe, with several usually found in each city or town in the United States, and many in other countries.
For US, US territories, Puerto Rico, and Canada based test-takers:
Go to http://www.actstudent.org/regist/centers/ and select your state, optionally the closest city, or a test center name if known.
For International test-takers:
Go to http://www.actstudent.org/regist/outside/ and select your country.